1. Field of the Invention
This invention relates generally to chemical mechanical planarization, and more particularly to methods of and apparatus for improved edge performance in chemical mechanical planarization applications by configuring a platen to control removal rate characteristics.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform Chemical Mechanical Planarization (CMP) operations, including polishing, buffing and cleaning. Typically, integrated circuit devices are in the form of multi-level structures formed on an underlying substrate. In the manufacture of such devices, the substrate with one or more such structures may be referred to as a wafer. Such wafers may include a semiconductor or other substrate, and structures such as those described below. For example, structures such as transistor devices having diffusion regions may be formed on the substrate. In subsequent levels, other structures such as interconnect metallization lines may be patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide.
As more metallization levels and associated dielectric layers are formed, there is an increased need to planarize the dielectric material of the wafer. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to variations in the surface topography. In other applications, additional structures such as metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization. Further applications include planarization of dielectric films deposited prior to the metallization process, such as dielectrics used for shallow trench isolation of poly-metal features.
CMP systems typically implement an operation in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of the wafer. The pad itself is typically made of polyurethane material, and may be backed by a supporting belt, for example a stainless steel belt. In operation, a liquid slurry is applied to and spread across the surface of the polishing pad. The pad moves relative to the wafer, such as in a linear motion across the wafer, and the wafer is lowered to the surface of the pad and is polished.
In the past, CMP operations have been performed using an endless belt-type CMP system, in which the polishing pad is mounted on two rollers, which drive the polishing pad in a linear motion. The wafer is mounted on a carrier head, which is rotated on a vertical axis. The rotating wafer is urged against the polishing pad with a force that is referred to as a down force FD. The down force results in a polishing, or first, pressure applied to the surface of the wafer. To resist the force FD, and the resulting first pressure, a platen is provided under the polishing pad and is vertically aligned with the carrier head and with the downwardly urged wafer. The platen is configured to cause a force to be applied upwardly on the polishing pad, and to thus cause a counter pressure PUP to be applied under the polishing pad. The counter pressure PUP is vertically aligned with the carrier head and with the downwardly urged wafer to resist the down force FD and the resulting first pressure. Slurry, such as an aqueous solution of NH40H or DI water containing dispersed abrasive particles, is introduced to the polishing pad upstream of the wafer. The process of scrubbing, buffing and polishing of the surface is performed by the polishing pad and slurry urged against the exposed surface of the wafer.
For reference, the wafer is said to have a peripheral edge, which is an edge of a perimeter that extends circularly around the wafer. Inwardly of the peripheral edge, there is an outer annular surface of the wafer. In a pre-polishing condition of the wafer, this outer annular surface may have an excessive and variable material thickness. This outer annular surface extends 360 degrees around the circumference of the wafer, and has a width that varies from tool-to-tool and process-to-process. Such width is radially symmetric and may have a value of from about 3 mm to about 45 mm, for an exemplary 300 mm wafer. For reference, the outer annular wafer surface has a portion referred to as a “leading” wafer surface portion (LWSP), which is adjacent to an intersection of a radius of the wafer and the peripheral edge of the wafer when such radius is parallel to the linear direction of the belt-type polishing pad during polishing. Because the wafer surface rotates clockwise during that linear polishing pad movement, successive portions of the outer annular wafer surface are the “leading” wafer surface portions LWSP at successive moments during such wafer rotation. Similarly, when one portion of the outer annular surface (that was an LWSP) has rotated 180 degrees from the location at which it was the LWSP, this former LWSP is now referred to as the “trailing” wafer surface portion (TWSP). Again, successive portions of the outer annular wafer surface are the “trailing” wafer surface portion TWSP at successive moments during such wafer rotation. For reference, the platen is also said to have a leading surface, or edge, LE, and a trailing surface, or edge, TE. The platen LE is adjacent to an intersection of the radius of the wafer (when that radius is parallel to the linear direction of the belt-type polishing pad during polishing) and a surface of the platen that is first under the linearly moving polishing pad. The platen TE is adjacent to an intersection of the radius of the wafer (when that radius is parallel to the linear direction of the belt-type polishing pad during polishing) and a surface of the platen that is last under the linearly moving polishing pad. The radial widths of the leading edge LE and trailing edge TE are not well-defined, but it is understood that such widths are less than or equal to the respective widths of the leading wafer surface portion LWSP and the trailing wafer edge portion TWSP.
Ideally, in a pre-CMP polishing condition, to-be-polished wafers are relatively flat. However, in many cases, the material profile of a to-be-processed wafer is not flat and as a consequence, excess material must be removed from some portions of the wafer. For example, if there is a need to remove such excess material from adjacent to the wafer peripheral edge, e.g., from the outer annular wafer surface, reference may be made to a “fast edge” process. Ideally, the fast edge process polishes the outer annular surface at a higher rate than that used to polish another portion of the wafer surface that does not have the excess material, for example. The many different rates of material removal from the same wafer ideally conform to a desired “material removal profile”. In this manner, and again ideally, the post-CMP processed wafer may have the desired degree of flatness.
In the past, to achieve the desired material removal profile, efforts have been made to provide the platen with fluid supply holes. The supplied fluid is generally air, and may be of many types, such as dry clean air. Reference is made herein to “fluid”, which includes such air. It is to be understood that other suitable fluids are included in the term “fluid”. In one such platen, these holes were arranged to define an outer group of concentric circular rings and multiple inner, groups of concentric circular rings, all of which were centered on the center of the platen, which is concentric with the central axis of the wafer. However, the fluid from these holes was not constrained. This lack of fluid constraint resulted in unacceptably high fluid usage. Furthermore, such platen was not fully amenable for use with all types of polishing belts. Specifically, results achieved with a flexible polishing belt were inferior to those achieved with a non-flexible belt.
Further efforts were made to reduce fluid usage and allow for the use of all types of polishing belts. A modified platen used a raised surface, hereafter referred to as a shim, in an effort to both restrict fluid usage and allow tuning of the material removal profile using either flexible or non-flexible polishing belts. The shim and a main platen surface cooperated with the polishing pad above the platen to define a fixed air pressure cavity. While this cooperation reduced the amount of air flowing from the chamber during CMP operations, difficulties were experienced in employing this platen configuration for achieving all polishing profile shapes, which are desirable to an end user. For example, in many instances, the pressure PUP within the cavity defined by the shim, the platen surface and the polishing belt, is largely constant. As a result of this largely constant pressure PUP, the material removal rate can also be largely constant. This largely constant material removal rate may be understood in terms of a characteristic of a curve that defines the removal rate of such described modified platen. Such a characteristic is that the constant removal rate is generally at a location around the center of the wafer. However, at a radial location, which corresponds to a region adjacent to the inner radius of the shim, that curve has an inflection point at which the relatively constant removal rate (due to the fixed-pressure in the cavity) suddenly changes. Thus, in the described modified platen, although there is a large area of uniform material removal surrounding the center of the wafer, the location of the inflection point is very closely adjacent to the peripheral edge of the wafer. The dimensions of the low pressure cavity of such modified platen are fixed in that the dimensions of the shim, the belt, and the platen are fixed, and such dimensions fix the size of the cavity. In this fixed dimension situation, once this modified platen is installed for CMP operations, it is not possible to significantly change the location of this inflection point. One unacceptable way of modifying the location of the inflection point would be to use shims that are adjustable to provide different shim diameters or shim widths. However, disadvantages of manufacturing cost and difficulties in use restrict the implementation of such an unacceptable configuration.
In review, to accommodate performing a removal of material that leaves a uniform surface of the wafer after the CMP operation, there is a need for an improved platen. This improved platen should reduce the amount of air that escapes from beneath the polishing pad in a manner which enables use of available low-cost polishing pads, and should provide an ability to position the inflection point at variable radial locations from the center of the platen during CMP operations. Further, the improved platen should be capable of achieving all of the material removal profiles that are desirable to an end user.